Application of sequential extraction

Microchemical Journal 72 (2002) 9–16
0026-265X/02/$ - see front matter ᮊ 2002 Elsevier Science B.V. All rights reserved.
PII: S0026-265X
Ž
01
.
00143-6
Application of sequential extraction and the ICP-AES method for
study of the partitioning of metals in fly ashes
Agnieszka Smeda, Wieslaw Zyrnicki*
Wroclaw University of Technology, Chemistry Department, Institute of Inorganic Chemistry and Metallurgy of Rare Elements,
Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
Received 11 June 2001; received in revised form 11 September 2001; accepted 14 September 2001
Abstract
In this work, the original BCR extraction scheme was modified and applied to study the partitioning of metals in
fly ashes. In the first step, the water-soluble fraction was investigated here. The next metal fractions were acid-
soluble, reducible, and oxidisable. Two kinds of coal fly ash certified reference materials were analysed. Metal
concentrations in the extracts were measured by inductively coupled plasma atomic emission spectrometry (ICP-
AES). The efficiency of the extraction process for each step was examined. The partitioning of metals between the
individual fractions was investigated and is discussed. ᮊ 2002 Elsevier Science B.V. All rights reserved.
Keywords: Sequential extraction; Fly ash; BCR scheme; Metal partitioning; Inductively coupled plasma atomic emission
spectrometry (ICP-AES)
1. Introduction
The interest in using coal in power plants to
produce electricity has not decreased in recent
years. Coal ash is the fossil-fuel combustion resi-
due from coal power plants. Deposits of fuel ashes
are a serious problem as a source of inorganic
pollution.
Knowledge of the chemical and physical prop-
erties of the ashes is important to assess the risk

of potential environmental mobility of toxic trace
metals. The availability and mobility of elements
*Corresponding author. Tel.: q48-71-320-2494; fax: q48-
71-328-4330.
E-mail address:
(W. Zyrnicki).
occurring in fly ashes depend on the physicochem-
ical forms of the elements.
Extraction methods are the tool for examination
of the element speciation. There are several extrac-
tion procedures reported in the literature, based on
different sequence schemes
w
1–7
x
and carried out
under various operating conditions
w
8–11
x
. The
approach developed by Tessier in 1979
w
12
x
and
so-called the BCR procedure
w
13
x

(proposed in
1993 by the European Community’s Bureau of
References — now The Standards, Measurements
and Testing Programme) are the most popular
schemes. The BCR scheme has been elaborated to
harmonise methodology and to enable the compar-
ison of results from different laboratories. The
BCR scheme has been widely applied to various
10 A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16
Table 1
Instrumental parameters and operating conditions for ICP-AES
Plasma
Generator frequency 40 MHz
RF power 1.0 kW
Nebuliser Cross-flow
Spray chamber Scott type
Monochromator
Type Czerny–Turner HR 1000
Focal length 1 m
Gratings 4320 and 2400 groovesymm
Argon flow rates
Plasma gas 13 dm ymin
3
Sheath gas 0.2 dm ymin
3
Nebuliser gas 0.3 dm ymin
3
Sample uptake 1.0 cm ymin
3
Elements and analytical lines (nm)

Al 396.152
Ba 233.527
Ca 317.933
Cr 267.716
Cu 324.754
Fe 259.924
Mg 285.213
Mn 259.373
Ni 221.647
Sr 407.771
Ti 334.941
V 292.402
Zn 202.548
matrices, e.g. sewage sludge
w
8,10,11,14
x
, different
soils
w
9,15–18
x
, and marine
w
6,19
x
and river sedi-
ments
w
2,3,17,20,21

x
.
So far, sequential extraction has rarely been
used to analyse fly ash samples
w
5,17
x
. Distribution
of Cd in fractions of the coal fly ash NBS 1633a
w
17
x
, and Cd, Cr, Cu, Pb, Zn and V in the fractions
of a brown coal
w
5
x
were recently studied with the
aid of atomic absorption spectrometry.
In the present study, the partitioning of metals
(Al, Ba, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Sr, V and
Zn) and B has been investigated using a sequential
extraction procedure with the aid of inductively
coupled plasma atomic emission spectrometry. The
BCR extraction protocol has been modified by the
introduction of leaching with deionized water as
the first step. Fractionation of the elements in the
coal ashes has been examined and is discussed.
2. Experimental
2.1. Instrumentation

The concentrations of metals in the extracts
were measured by inductively coupled plasma
atomic emission spectrometry (ICP-AES). A Job-
in-Yvon sequential ICP spectrometer (JY 38S)
was used for measurements. The instrumental
operating conditions are shown in Table 1.
For extraction, a horizontal, mechanical water-
bath shaker was employed. A centrifuge was used
for separation of the solid phase from the extrac-
tion liquid.
2.2. Samples
Two certified reference materials were examined
here: CTA-FFA-1 (Fine Fly Ash CTA-FFA-1;
Polish Certified Reference Material for multiele-
ment trace analysis) and ENO No.12-1-01 (major
and trace elements in brown coal fly ash ENO
No.12-1-01; Slovak Certified Reference Material).
2.3. Reagents
Standard solutions were prepared by dilution of
a multielement standard solution (Merck, 1000
mgycm ). All reagents were at least of analytically
3
pure grade. Hydroxylamine hydrochloride solution
(PPH-POCh, Gliwice, Poland) was prepared prior
to use. For pH adjustments, nitric acid (65%,
Merck, Germany) was used. All solutions were
prepared with deionized water (18.3 MV cm
resistivity, Barnstead Easy pure RF series 703).
Glass- and plasticware were cleaned in 10%
HNO in an ultrasonic bath and then rinsed a few

3
times with deionized water.
2.4. Procedure
The four-step extraction procedure shown in Fig.
1 was used here. The following metal fractions
were investigated: water-soluble forms removed by
water (deionized); acid-soluble forms associated
with carbonates; reducible forms associated with
oxides and hydroxides of Al, Mn and Fe; and
oxidisable forms associated with organic matter or
11A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16
Fig. 1. Schematic diagram of the sequential extraction
procedure.
sulfides (for more details see
w
19
x
). Five samples
(1g) of each ash were placed in separate 50-ml
polypropylene tubes. For each step of the extrac-
tion, a blank sample (without ash) was carried
out. There was no delay between adding the
extractants and beginning the shaking. The extracts
were stored in polypropylene bottles and kept at 4
8C before measurement.
Between each stage, the residues were washed
with 20 ml of deionized water, followed by shaking
for 20 min and centrifugation. The supernatant
(washing solution) was discarded, taking care not
to lose any of the solid residue.

Digestion of the residue is not specified in the
BCR protocol, so the residual fraction was calcu-
lated as the difference between the total element
concentration and the sum of all previous steps. In
this part of our study, we strictly followed the
original BCR procedure.
3. Results and discussion
The main elements in the brown coal ash (ENO
No.12-1-01) with certified concentration above 1%
were Mg (1.2%),K(1.7%),Ca(3.4%),Fe
(7.5%),Al(10.8%) and Si (25.7%). Certified
values for the concentration of the main elements
in bituminous coal ash were: 1.6% Mg; 2.2% K;
2.2% Na; 2.3% Ca; 4.9% Fe; 14.8% Al; and
22.5% Si. No information on carbon and oxygen
contents was available. The total concentrations of
sulfur and boron (0.25% and 291 mgyg, respec-
tively) were known only for the brown coal ash.
X-Ray diffraction analysis was employed to
determine the crystalline compounds in fly ashes.
The proportion of crystalline components in both
samples varied, depending on the type of coal.
The main crystalline phases in the brown coal ash
were anhydrite, quartz, hematite, labradorite and
magnesioferrite. Small amounts of mullite and
aragonite were also found. The bituminous coal
fly ash contained mainly mullite, quartz and crys-
talline Ca Al O . Anhydrite, magnesioferrite, lab-
326
radorite, aragonite, hematite, lime and periclase

were also identified. Large amounts of amorphous
phases were found in both ashes.
Results of the modified BCR extraction proce-
dure for the main and trace elements are presented
in Table 2 and Figs. 2 and 3. For each material
and extraction, five samples were simultaneously
analysed to determine the precision of the meas-
urements. The RSD values varied over a wide
range and depended on the element and the extrac-
tion stage. Very good precision, usually in the
range 1–15%, was achieved for most of the
elements analysed here. In a few cases, the RSD
values obtained were approximately 60%. Such
low precision was observed for measurements of
the water-soluble fraction, in which some metal
12 A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16
Table 2
Metal partitioning obtained by sequential extraction
Element Step 1 (mgykg) Step 2 (mgykg) Step 3 (mgykg) Step 4 (mgykg) Total content (mgykg)
FFA BCA FFA BCA FFA BCA FFA BCA FFA BCA
Al 23"11 127"41 720"27 692"55 1228"119 893"110 33"18 36"13 14.87"0.39
a
10.8"0.3
a
(1.5=10 )
y2
(1.2=10 )
y1
(4.8=10 )
y1

(6.4=10 )
y1
(8.3=10 )
y1
(8.3=10 )
y1
(2.2=10 )
y2
(3.3=10 )
y1
B 407"45 23.9"3.1 120"10 55.5"1.6 15.0"2.2 12.6"1.8 6.11"1.32 5.25"0.43 – 291"39
(8.2)(19)(4.3)(1.8)
Ca 4910"400 2450"70 4960"390 6590"160 694"92 1650"220 483"150 590"80 2.29
a
3.42"0.24
a
(21)(7.2)(22)(19)(3.0)(4.8)(2.1)(1.7)
Fe 0.15"0.12 1.49"0.56 9.86"1.32 30.8"3.3 770"68 768"77 9.49"2.15 10.1"6.8 4.89"0.14
a
7.49"0.11
a
(2.5=10 )
y4
(2.0=10 )
y3
(2.0=10 )
y2
(4.1=10 )
y2
(1.6)(1.0)(1.9=10 )

y2
(1.3=10 )
y2
Mg 53"15 190"9 5400"80 1020"30 527"84 229"36 108"16 72.7"5.7 1.55
a
1.17"0.05
a
(3.4=10 )
y1
(1.6)(35)(7.8)(3.4)(2.0)(6.9=10 )
y1
(6.2=10 )
y1
Values in parentheses represent distributions in %. FFA, Fine Fly Ash Reference Material CTA-FFA-1; BCA, Brown Coal Ash Reference Material ENO No. 12-1-
01.
Values in wt.%.
a
13A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16
Fig. 2. Comparison of metal distributions for the different fractions (in %): s1, water-soluble; s2 acid-soluble; s3, reducible; and
s4, oxidisable fractions.
concentrations were very low or close to their
detection limits.
For most elements, the distribution of metals in
the extracts was similar for both ashes. Notable
differences observed for some elements were con-
nected with the nature of the materials.
Analysis of the content of the major elements
reported by the supplier of the ashes indicates that
Ca and Mg should be in silicate and aluminosili-
cate forms. X-Ray diffraction spectra suggest that

the Ca and Mg silicates are amorphous. Significant
differences appear in the case of Ca and Mg for
the ashes analysed. The extraction efficiency of
Ca was considerably higher for bituminous than
for brown coal ash. The quantity of calcium in the
water-soluble fraction was approximately one order
of magnitude higher than the magnesium content.
In the second fraction (acid-soluble and associated
with carbonates), the Ca and Mg concentrations
were meaningful and similar. More than 60% of
the Ca and Mg was in the residue. Of the other
major elements, Al and Fe behaved very similarly
and remained in the deposits after extraction. For
both ashes, nearly identical results were observed.
More than 98% of the aluminium and iron were
identified in the residual fraction. Al was present
14 A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16
Fig. 3. Comparison of metal distributions for the different fractions (in %): s1, water-soluble; s2, acid-soluble; s3, reducible; and
s4, oxidisable fractions.
in different forms, mainly in compounds with
silicon as crystalline mullite and with Ca as
Ca Al O . Fe occurred in the ashes in the form of
326
oxides, such as magnetite or hematite. It is very
likely that Fe is also present in amorphous forms.
Boron, which remained in significant quantities
in the residue (brown coal ash), can be both in
borate and boride forms.
Of the trace elements, chromium, boron and
strontium were relatively easily extracted by deion-

ized water. Zinc and titanium in the bituminous
coal ash were found in forms that are not easily
soluble in water. Their concentrations in the water
extracts were below or very close to their detection
limits. For brown coal ash, Ti and Zn were also
practically not extracted — this fact indicates that
these elements are not released under typical envi-
ronmental conditions. In the acid-soluble fraction,
nearly 10% of chromium, copper and strontium
were found to be associated with carbonates. The
reducible fraction contained a considerable amount
of vanadium (17% for bituminous coal ash and
9% for brown coal ash). The extraction efficiency
for other elements did not exceed 5% of their total
15A. Smeda, W. Zyrnicki / Microchemical Journal 72 (2002) 9–16
concentrations. For chromium and vanadium, high
values were found in the oxidisable fractions —
they were extracted in 8–9% for Cr and 5–7% for
V. A large portion of the elements analysed was
found in the residual fraction. For titanium, the
metal was practically not extracted. Less than 2%
of its total content was found in fractions 1–4.
Barium, copper and nickel were also hardly
extracted from the ashes (85–93% in the residue).
This indicates that these elements are concentrated
in the undissolved aluminosilicate matrix.
Due to existence of many different sequential
extraction procedures, it is very difficult to com-
pare the results obtained by various authors. On
the other hand, not many such studies have been

reported so far for coal fly ashes. Comparison of
our results for Cr, Zn and V with those obtained
by Bodog et al.
w
5
x
shows that the content of these
´
metals in the residual ash fraction could be signif-
icantly different. On the other hand, the Cr, Zn
and V distributions in the reducible fractions and
bound to MnyFe oxides of various ashes are
comparable.
So far, only one reference material (CRM 601,
lake sediment) certified for metals extractable by
the BCR procedure has been produced. Only Cd,
Cr, Cu, Ni, Zn and Pb contents have been certified.
No such a reference material is available for fly
ashes. Thus, a reference material for the sequential
extraction has not been used here.
4. Conclusions
The ICP-AES method is very applicable to study
of a multielement extraction process and metal
partitioning in materials such as fly ashes.
The sequential extraction procedure reveals
much more information about elements investigat-
ed than data obtained from measurements of their
total concentrations. Development of the BCR
procedure (with water extraction as the first step)
is recommended, because the extraction of water-

soluble species yields very important information
necessary to evaluate the risk of environmental
pollution by dumps of coal ashes.
The BCR procedure enabled comparison of
metal partitioning in various types of environmen-
tal samples (soil, sediments, and sewage sludge).
However, for better understanding of the speciation
and partitioning of metals in specific materials,
such as fly ashes, more advanced tests should be
carried out. For example, the presence of sulfur as
various metal sulfides (such Cu S, CuS, FeS,
2
Fe S and ZnS) would be expected in fly ashes.
23
Therefore, it seems to be necessary to add more
extraction steps with properly selected extractants,
including more aggressive reagents.
At the present state of knowledge, it is very
difficult to explain in detail both the distribution
and speciation of metals in materials such as fly
ashes. Coal ashes are generated in a very aggres-
sive combustion process and comparison of the
ashes with other materials, such as soil and sedi-
ments, shows that heavy metals are not removed
as easily as from these materials.
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